This invention relates generally to heat pipes, and more specifically to methods for making sintered metal heat pipe wicks.
Heat pipes use successive evaporation and condensation of a working fluid to transport thermal energy, or heat, from a heat source to a heat sink. Because most fluids have a high heat of vaporization, heat pipes can transport in a vaporized working fluid very large amounts of heat. Further, the heat can be transported over relatively small temperature differences between the heat source and heat sink. Heat pipes generally use capillary forces through a porous wick to return condensed working fluid, or condensate, from a heat pipe condenser section (where transported thermal energy is given up at the heat sink) to an evaporator section (where the thermal energy to be transported is absorbed from the heat source).
Heat pipe wicks are typically made by wrapping metal screening of felt metal around a cylindrically shaped mandrel, inserting the mandrel and wrapped wick inside a heat pipe container and then removing the mandrel. Thus constructed heat pipe wicks are particularly susceptible to developing hot spots where the liquid condensate being wicked back to the evaporator section boils away and impedes or blocks liquid movement. Such hot spots usually occur at gaps between the wick and the inside wall of the container, and also at nonhomogeneous locations, such as dense areas or relatively large voids, in the wick structure itself. These gaps and other nonhomogeneties are nearly impossible to avoid using conventional wick construction methods.
Gaps between the container and wick arise primarily from difficulties in attaching or adhering the wick structure to the inside wall. The wick is generally force fit inside the container so that residual internal stresses hold it in place. Unfortunately, over time the high temperatures from operation of the heat pipe anneal the wick, which reduces the internal stresses and allows the wick to pull away from the inside wall. Attempts to use bonding agents or cements to bond the wick structure to the inside wall meet with the difficulty, shared with the binders used to make felt metal, that typical bonding agents disintegrate at high pipe temperatures.
Nonhomogeneties are inherent in most wick structures. Prior art attempts to make a more homogeneous, or more uniformly nonhomogeneous, wick structure, and also to avoid the problems caused by annealing, include the use of sintered metal heat pipe wicks. Sintered metal is attractive as a wicking material because it is easily formed into a variety of shapes and the prior art has developed a variety of methods for making sintered metal of varying porosity and differing morphologies. Prior art sintered metal wicks have been made primarily by filling powered metal into the space between a mandrel and a heat pipe container and then heating the powder to sinter together the individual particles and make a porous wick. The mandrel, having been previously surface treated to aid separation, is then removed from inside the sintered wick. Unfortunately, these methods for making sintered metal wicks produce wicks that still suffer from nonhomogeneties and from an imperfect fit between the inside surface of the container and the wick. A particular problem with such methods is that it is very difficult to keep an even spacing between the mandrel and inside wall to produce a wick of even thickness.
Another attempt by the prior art to avoid wick material problems includes using, instead of wick material, longitudinal grooves in the heat pipe container inside wall to wick condensate back to the evaporator section. Grooves and other structural wicking aids, however, are used most advantageously in combination with porous wicks.
Thus it is seen that there is a need for improved heat pipe wicks that avoid both nonhomogeneties in the wick material and gaps between the container inside wall and the wick.
It is, therefore, a principal object of the present invention to provide an improved method for making sintered metal heat pipe wicks that are homogeneous and firmly attached without gaps to the inside wall of the heat pipe container.
It is another object of the invention to provide a method for making sintered metal heat pipe wicks that are of exceptionally accurate and even thickness over the heat pipe container inside wall.
It is a feature of the invention that it produces sintered metal heat pipe wicks of uniformly varying pore sizes.
It is another feature of the invention that it uses a precursor wick material comprising a slurry that can be poured over a variety of different heat pipe container inside wall shapes and attachments so that the final heat pipe wick provides a complete cover for such attachments without substantially interfering with the operation of the wick.
It is an advantage of the invention that its use of a pourable slurry permits making a wick in tight spaces and around corners so that it can be used to manufacture wicks of complex shapes.